JP4492364B2 - Method for producing electrogalvanized steel sheet with excellent plating appearance - Google Patents

Description

  The present invention relates to a method for producing an electrogalvanized steel sheet, more specifically, a method for producing an electrogalvanized steel sheet having an excellent plating appearance.

  Conventionally, electrogalvanized steel sheets have often been used as coating bases by taking advantage of their excellent corrosion resistance. In recent years, the electrogalvanized steel sheet has been subjected to chemical conversion treatment such as chromate treatment and transparent resin coating to give the required properties such as corrosion resistance and fingerprint resistance. The tendency to use it as it is (unpainted) is increasing.

  Electrogalvanized steel sheets are made by degreasing and pickling the plating base plate to clean and activate the surface, and then electroplating zinc using a plurality of plating tanks with the steel sheet as the cathode in a plating solution containing zinc ions. Then, if necessary, a chemical conversion treatment is performed after electroplating.

  Since the surface appearance of the steel sheet after chemical conversion treatment depends on the plating appearance, it is necessary that the plating appearance is good. It is important that the plating appearance not only has no original plate defects and plating defects, but also has a good appearance color tone. Since the electrogalvanized film is almost achromatic, the color tone of the plating appearance depends on the brightness. As the appearance color tone, a bright appearance is often preferred. Therefore, it is desirable that the brightness can be increased.

Patent Document 1 proposes that the lightness of the steel sheet surface after plating is improved by containing 0.01 to 10 ppm of Tl in the galvanizing bath.
Japanese Patent Laid-Open No. 9-195082 (page 2)

  According to Patent Document 1, it is possible to significantly increase the brightness, but there is a problem that the brightness fluctuation cannot be reduced because the brightness changes greatly with a slight change in the Tl content. Furthermore, the addition of a new component to the plating bath not only complicates the management of the plating bath, but also raises the problem of increasing the cost of the plating solution. In addition, the inclusion of a new component in the plating bath may affect the quality other than the color tone.

  In view of the above problems, an object of the present invention is to provide a method for producing an electrogalvanized steel sheet capable of increasing the brightness without adding a new component to the plating bath.

  The gist of the present invention for solving the above problems is as follows.

In the first invention, when producing an electrogalvanized steel sheet having a zinc coating amount of 10 to 40 g / m ² by electrogalvanizing the steel sheet a plurality of times in a plurality of plating tanks, at least one or more plating tanks are used. A pre-plating step of performing steel plate electrogalvanization, and a post-plating step of performing steel plate electrogalvanization in at least one or more plating baths, and at least one bath between the pre-plating step and the post-plating step It has a dipping process of dipping through a plate above the plating tank without passing a steel sheet to galvanizing solution, zinc coating weight of the pre-stage plating process w1 (g / m ^2), zinc coating weight of the subsequent plating step w2 (g / m ² ) is a method for producing an electrogalvanized steel sheet characterized by satisfying 3.0 ≦ w1 / w2 ≦ 4.5 .

A second invention is a method for producing an electrogalvanized steel sheet according to the first invention , wherein the dipping step dissolves 0.1 to 5.0 g / m ^{2 of} plated zinc.

A third invention is the method for producing an electrogalvanized steel sheet according to the first invention , wherein the dipping step comprises immersing the steel sheet in a zinc plating solution having a pH of 1 to 3 for 1 to 20 seconds.

  According to the present invention, the brightness of the plated steel sheet surface can be effectively increased without reducing productivity. Moreover, in this invention, since a new component is not added in a plating bath, there is no possibility of affecting the plating quality, such as blackening.

  The present inventors investigated in detail the influence which the operating conditions in an electrogalvanization line have on the lightness of the plated steel sheet. As a result, it has been found that the brightness increases by providing a step of immersing and passing the steel plate in the plating solution without energization in the middle of the electrogalvanizing step. The present invention is based on this finding.

  Hereinafter, the manufacturing method of the electrogalvanized steel sheet of this invention is demonstrated.

Usually, the coating adhesion amount of the electrogalvanized steel sheet is 10 to 40 g / m ² . In the present invention, a method for improving the lightness of the electrogalvanized steel sheet in such a coating adhesion amount range was studied.

Usually, the electrogalvanized steel sheet is subjected to electrogalvanization a plurality of times using a continuous electroplating facility equipped with a plurality of plating tanks to obtain a desired galvanized coating amount. In the conventional manufacturing method, electroplating is performed with substantially the same plating current density in each of a plurality of plating tanks. In the present invention, when producing an electrogalvanized steel sheet having a zinc coating amount of 10 to 40 g / m ² by electrogalvanizing a steel sheet a plurality of times in a plurality of plating tanks, at least one tank is provided during the plating process. In the above plating tank, the steel plate is immersed in the galvanizing solution without being energized. That is, the plating step is at least one tank between a pre-plating process in which steel plate electrogalvanizing is performed in at least one plating tank and a post-plating process in which steel plate electrogalvanizing is performed in at least one plating tank. In the above plating tank, the steel plate is immersed in the galvanizing solution without being energized. The brightness of the plating surface can be increased by providing an immersion process in which the steel sheet is immersed in the zinc plating solution without energization between the former plating process and the subsequent plating process.

  The reason why the lightness of the surface is increased by providing a dipping process in which the steel sheet is dipped and passed through the zinc plating solution without energization in the middle of the plating process is not clear, but is estimated as follows.

  When zinc plating is performed at a high current density, the formed zinc crystals become coarse crystals. When plating with a high current density is repeated in a plurality of plating tanks, the coarse zinc crystals described above are stacked in layers, resulting in coarser zinc crystals, thereby reducing the surface brightness.

  In the middle of the plating process, by providing an immersion process in which the zinc plating solution is immersed in a non-energized manner, the outermost layer portion of the coarse zinc crystal generated in the previous plating process is dissolved and the unevenness thereof is relaxed. In the latter plating process, fine crystals are generated by plating on the plating surface where the unevenness is relaxed, and the brightness of the surface is improved.

If the amount of zinc plating produced in the pre-plating process is too large, the resulting zinc crystals will become too coarse, and in the next immersion process, the action of relieving the unevenness of the outermost layer of the coarse zinc crystals will be insufficient. Thus, fine crystals are not generated in the subsequent plating step, and the effect of improving the brightness is reduced. Also, in the dipping process, even if the unevenness of the rough zinc crystals generated in the previous plating process is relaxed, if too much zinc is generated in the subsequent plating process, coarse crystals will be generated again, and the brightness will be increased. The effect of improving decreases. For these reasons, in order to improve the brightness, the zinc plating adhesion amount w1 (g / m ² ) in the former plating step and the galvanization adhesion amount w2 (g / m ² ) in the subsequent plating step are 1 ≦ w1 It is preferable to satisfy / w2 ≦ 5.

  Usually, the current density of each plating tank is often set to substantially the same current density. Since the plating adhesion amount has a corresponding relationship with the energization time, the plating adhesion amount may be defined based on the energization time of each process, instead of the previous and subsequent plating adhesion amounts. In other words, if the energization time in the pre-plating process is too long, the zinc crystals generated in the pre-plating process become too coarse, so in the next soaking process, the outermost layer portion of the coarse zinc crystals is dissolved to relieve the unevenness. The effect becomes insufficient, and the effect of improving the brightness after the subsequent plating step is reduced. Also, in the dipping process, even if the unevenness of the rough zinc crystal generated in the previous plating process is relaxed, if the plating time in the subsequent plating process is too long, a rough crystal will be generated again, improving the brightness. The effect is reduced. For these reasons, in order to improve the brightness, the plating energization time t1 of the pre-plating process before the dipping process and the energization time t2 of the post-plating process after the dipping process satisfy 1 ≦ t1 / t2 ≦ 5. It is preferable.

In the dipping process, it is preferable to dissolve 0.1 to 5.0 g / m ^{2 of} plated zinc. When the amount of zinc dissolved is less than 0.1 g / m ² , the action of relieving the unevenness of the surface by dissolving the outermost layer portion of the coarse zinc crystal generated in the previous plating process becomes insufficient, and zinc generated in the subsequent plating process The effect | action which refines | miniaturizes a crystal | crystallization falls. Even if the amount of zinc dissolved exceeds 5.0 g / m ² , the effect of improving brightness is small, and not only the amount of plating power used is increased, but also plating unevenness occurs on the surface of the steel sheet and the appearance may be inferior.

  The zinc dissolution amount in the dipping process can be determined by examining the corresponding zinc dissolution amount by changing the plating solution immersion conditions, for example, the plating solution pH, immersion time, and the like, thus ensuring the zinc dissolution amount. The immersion conditions can be easily determined. For example, in the case of a sulfuric acid zinc plating solution, the aforementioned zinc dissolution amount can be obtained by setting the pH within a range of 1 to 20 seconds in a zinc plating solution having a pH of 1 to 3.

After cold rolling, an ultra-low carbon steel plate (plating original plate) having a thickness of 0.8 mm and a width of 1219 mm manufactured by annealing and temper rolling was prepared. The steel sheet is charged into an electroplating facility equipped with a plurality of plating tanks and a continuous electrogalvanizing line equipped with two roll coaters (first roll coater and second roll coater) downstream of the steel plate in the sheet passing direction. Then, after degreasing and pickling by a normal method, using a sulfuric acid acidic zinc plating bath containing 400 g / l of zinc sulfate and 50 g / l of sodium sulfate, temperature: 60 ° C., pH: 1.4, The number of intermediate plating tanks to be used was changed, and galvanization was performed so that the amount of galvanization after the post-stage plating step was 20 g / m ² . The current density of each plating tank was set to the same current density so that the amount of galvanization after the subsequent plating step was 20 g / m ² . The zinc dissolution amount at the time of immersion was calculated in advance from the relationship between the immersion time and the zinc dissolution amount. Table 1 shows the steel sheet thickness, line speed, plating bath usage conditions, and plating conditions.

Subsequently, with a first roll coater, a lower layer coating treatment solution containing 0.01 mol / l each of primary phosphoric acid, colloidal silica, and Mg was applied so that the reference adhesion amount in terms of P was 45 mg / m ² . After heating and drying, a lower phosphoric acid-containing film is formed, and then an organic resin solution containing an epoxy resin is applied with a second roll coater so that the reference film thickness upon drying is 1 μm, followed by heating and drying. An upper organic resin-containing film was formed.

  The lightness of the surface-treated steel sheet in which the electrogalvanized layer produced above and a multilayered film composed of a phosphoric acid-containing film on the lower layer and a resin film on the upper layer was measured was measured. The lightness was evaluated by a lightness index L value measured by a method (condition d, Hunter method) defined in JIS Z 8722. The measurement results are also shown in Table 1.

  The example of the present invention has higher brightness (L value) and improved appearance color tone as compared with the conventional example plated in the entire plating tank.

  The present invention can be used as a method for producing an electrogalvanized steel sheet having high brightness.

Claims (3)

When producing an electrogalvanized steel sheet having a zinc coating amount of 10 to 40 g / m ² by electrogalvanizing the steel sheet several times in a plurality of plating tanks, the steel sheet electrogalvanizing is performed in at least one or more plating tanks. A pre-plating step to be applied, and a post-plating step of applying steel plate electrogalvanization in at least one or more plating baths, and at least one or more plating baths between the pre-plating step and the post-plating step steel sheet have a soaking step of immersing through the plate in the non-energized to the galvanizing solution, the pre-stage plating zinc coating weight step w1 (g / m ^2), zinc coating weight of the subsequent plating step w2 (g / m ² ) is a method for producing an electrogalvanized steel sheet , wherein 3.0 ≦ w1 / w2 ≦ 4.5 is satisfied . The method for producing an electrogalvanized steel sheet according to claim 1 , wherein the dipping step dissolves 0.1 to 5.0 g / m ^{2 of} plated zinc. 2. The method for producing an electrogalvanized steel sheet according to claim 1 , wherein the dipping step comprises immersing the steel sheet in a zinc plating solution having a pH of 1 to 3 for 1 to 20 seconds.